In a conventional electrical discharge machining apparatus, machining is generally performed in accordance with previously set machining conditions. In other words, machining is generally executed under initially set machining conditions without changing machining conditions, including voltage applied between an electrode and a workpiece, servo voltage which determines the electrode feeding speed, capacitance in the case where the apparatus uses a capacitor discharge power source, on/off-time for the voltage applied between the electrodes, etc., during the machining operation.
A method in which machining conditions are changed depending on a discharge state between electrodes during machining operation is disclosed in Japanese Patent Publication No. 63-17568, for example. In the machining method described in this document, the machining conditions are changed in a manner such that a storage device is provided beforehand with a data table stored with data of several kinds, the data stored in the storage device are selected on the basis of detected data from a detecting device for detecting the discharge state, and new machining conditions are determined by executing an arithmetic operation for the selected data.
According to the method described above, the data are selected from the data table of the storage device in accordance with the detected data, and the machining conditions are changed depending on the selected data, so that the change of the machining conditions are restricted by the contents of the data table. Since the machining conditions can be changed only by stages, the controllability is not satisfactory. If the data stored in the table are increased in quantity so that the machining conditions can be changed to reflect fine differences between the detected data, this will cause not only a problem that the storage capacity of the storage device is depressed but also a problem such as the drop of the processing speed.